1. Field of the Invention
This invention pertains to a light source for a lithographic device for use in semiconductor manufacturing operations. More particularly, the invention pertains to an array of small lasers as the light source for a lithographic exposure device for use in manufacturing semiconductors.
2. Description of the Related Art
Semiconductor fabricators are motivated to increase semiconductor performance and reduce manufacturing costs, factors that are greatly affected by the semiconductor fabrication process. Optical lithography is a preferred fabrication process for producing semiconductors. An optical lithographic system includes an exposure device, a mask, and resist.
Semiconductor performance is affected by the size of the circuits on the substrate. Thus, a goal in semiconductor fabrication is to create a small feature size, that is, a minimum size of an object formed on a semiconductor substrate. The minimum feature size that can be fabricated by an optical lithographic process is proportional to the wavelength of the light source of the exposure device. Previous optical lithographic systems used light from a mercury lamp light source having a wavelength of 436 to 365 nm. Currently, there is much research to optimize optical lithographic systems using a laser light source having wavelengths less than 250 nm in order to create a smaller minimum feature size.
While lasers can produce light energy having a short wavelength, other problems occur when a laser light source is used to project a mask pattern onto a substrate.
When a linearly polarized laser light source illuminates a mask pattern having linear portions, the width of linear portions oriented parallel to the direction of linear polarization will be different than the width of linear portions that are oriented orthogonally to the direction of the laser polarization. This is a disadvantage. The analysis and experimental results concerning this problem are described by Asai et al. (S. Asai, I. Hawyu, and M. Takikawa, Resolution Limit for Optical Lithography Using Polarized Light Illumination, Jpn. J. Appl. Phys., Vol. 32, 1993, pp. 5863-5866.)
As noted, early exposure devices used a mercury lamp as the light source. The mercury lamp light is nonpolarized, thus the line widths of the semiconductor features were uniform regardless of the orientation of the linear features of the mask.
An exposure device using a UV laser generates polarized light. In order to solve the aforementioned problem of varying line widths, prior art laser exposure devices used a birefringent optical element to make the overall light output from the light source nonpolarized. However, the birefringent element reduces the available power from the laser light source to the substrate.
A prior reference discloses a low-coherence laser light source having a plurality of miniature UV lasers in a bundle that is used as a UV light source for the exposure device.
For the laser light source which has a plurality of UV lasers set as a bundle, the outputs of the various UV lasers are linearly polarized. However, as described in the aforementioned reference, they do not interfere with each other, because they have low coherence. Consequently, because the laser beams emitted from the various lasers overlap, it is possible to obtain a light source for which the illumination becomes uniform, and the coherence level is lowered so that speckling is alleviated.
However, when the laser beams emitted by the plurality of lasers overlap, if all of the polarization directions of the laser beams are the same, the polarization direction of the overlapping illuminating light becomes linearly polarized light, and the aforementioned problem occurs wherein lines having different orientations have different line width on the substrate.